Fireflies in Mantra render direct reflection

[bentway23]

I'm having a devil of a time trying to get rid of fireflies in a render (files and an .exr frame attached). I've isolated it to the direct reflection pass (the hotspots along the nose in the pic), and Nuke tells me they are way hot superbrights. Doing the usual Google fixes (working with diffuse limit, boosting reflection quality and overall render quality, &c.) haven't changed anything. In the scene I have three lights and surprisingly it's not the environment light that's causing the problem (I isolated the lights and also boosted the sampling quality on the environment light), but the key light (a spot light, but I tried distant and area lights with the same result).

This is my first time dabbling with Substance Painter, which I used for a quick not-too-boring look, and it looks like the problem might be with the roughness or metallic maps. They're .tgas, and open fine in Photoshop. If I turn off all maps and boost the roughness the hotspots are less hot and spotty, but I don't think that's actually causing the problem.

I'm sure this is something easy and stupid. Thanks for any assistance!

(Credit where due: the original bust is from a photoscan by Ryan Baumann on Sketchfab.)

caligulaFireflies.zip

[toadstorm]

If you're seeing noise in direct reflections, the best place to crank the samples you want is the Sampling Quality parameter on the light. You could also increase pixel samples, but this will multiply samples for everything, which you probably don't need in this case.

[bentway23]

There's no sampling quality option on the distant and spotlights--it's greyed out. A pixel sample boost was the first thing I tried, and didn't change anything.

Also, perhaps worth noting--if all the non-key lights are turned off, and the key light exposure cranked waaay down to almost complete darkness, the fireflies are still there. This is the same wither rendering through PBR or ray tracing.

Edited March 29, 2020 by bentway23

[toadstorm]

Took a closer look at your scene. There's a few things that probably need adjustment. One, your roughness map has very hard edges between different roughness values, and is also very noisy. If you blur the roughness map slightly, you can resolve some of these artifacts. Two, you're using spot lights, which are infinitely small light sources, so you're not going to get very nice soft rolloffs on your highlights. I'd just use small-ish area lights instead of spot lights here, and then you'll have control over the sampling quality. 

[bentway23]

You are my favorite person of the day. The problem was indeed that those "hotspots" were actually reflections of the infinitely small spotlight, as I saw when I replaced it with an area light or moderate size. The solution--find a better HDR and kill all the other lights.

Going to revisit the roughness map, but changing up the light did it. Thanks for your time!!